For applications where input voltages can exceed input ratings, the easiest option is to choose a CSA (or even a hall-based or isolated current sensor) that can survive the voltage; however, this is not always possible. When this is the case, the best practice is to use the following input protection scheme shown in Figure 3-1.

Figure 3-1 Input Protection for EOS

In Figure 3-1, D1 and D2 need reverse-breakdown voltages before the Absolute Maximum V_CM Voltage of the CSA. Ideally, voltages at input pins are clamped before the maximum V_CM rating. If D1 and D2 clamp after the V_CM rating, the R_Protect2 resistor can limit the rest of current into ESD pin to < 5mA peak. General equation to determine R_Protect2 is shown in Equation 1.

Figure 3-1 shows unidirectional diodes and not bidirectional TVS diodes because during negative voltage event the clamping diodes breakdown with their forward bias characteristic, which is much better than a larger breakdown voltage. The resulting forward bias voltage (V_F) can probably be less than the -0.3V minimum V_CM rating for low-side CSAs. Thus, for negative voltage surges, R_Protect2 can be necessary to still limit current according to Equation 2.

If the CSA is going to be rated to system level standards such as IEC61000-4-2, then D1 and D2 need to be TVS diodes. For any less stringent and/or slower EOS events, D1 and D2 can be Zener diodes that are rated to withstand the current draw for however long the EOS can occur. It is best to conservatively assume that the EOS condition can last indefinitely.

R_Protect1 helps limit current mostly into D1 and D2 (and thus reduces required power ratings). However, there can be a tradeoff with circuit offset error over temperature due to D1 and D2 leakage currents. Mismatch in these leakage currents, along with mismatch in the R_Protect1 resistors can create variable voltage drops in the R_Protect1 resistors, which can cause input offset error.

The input capacitors (C_CM and C_DIFF) can attenuate fast voltage surges increase robustness to system level testing and noise immunity.

Most CSAs (except for high-input impedance ones such as INA190 or INA186) can require total resistance into an input (R_Protect1+R_Protect2) to be less than 10-Ohm to avoid any noticeable change in error. However, there are methods to bound this error. Refer to this Input Resistance Error for Current Sense Amplifiers, user's guide for more information.

Additional testing of a CSA against IEC surges can be found Transient Robustness for Current Shunt Monitor, reference design.